The present invention relates to ground distance protective relays, and more particularly to a ground distance relay which employs a phase comparator method of measurement.
As a result of advancements in solid state technology, static protective relays have been developed. Exemplary circuits for phase comparator static relays are disclosed in my U.S. Pat. No. 4,034,269, entitled "Protective Relay Circuits" issued July 5, 1977. This patent is assigned to the assignee of the present application and is hereby incorporated by reference in the present application. Briefly, U.S. Pat. No. 4,034,269 discloses circuit means for discriminating between low level and high level faults such that substantially no delay occurs in the detection of high level faults.
In the phase comparator method of measurement, the relay operates by measuring the angle between two or more phasors derived within the relay. The phase comparator method of measurement allows the formation of characteristics, such as the MHO and offset MHO characteristics, which are substantially the same as the corresponding characteristics of conventional electromechanical relays.
FIGS. 6 and 7 of U.S. Pat. No. 4,034,269 disclose exemplary circuits suitable for a distance relay wherein the phase angle comparator method of measurement is employed. Referring now to FIG. 1A of the present application, a portion of the FIG. 6 circuit of U.S. Pat. No. 4,034,269 is shown with several modifications required for use as a ground distance relay. For purposes of clarity, where possible, like reference numerals have been employed to represent like elements. Briefly, in a phase A ground distance relay of FIG. 1A, a coincidence detector 286 functions to detect phase coincidence between a polarizing signal (V.sub.BC 90.degree.) and an operating signal (I.sub.A Z.sub.R +KI.sub.0 Z.sub.R -V.sub.AN). These signals will be discussed more fully in the Detailed Description of the Invention in the present application. A timer 288 measures the duration of phase coincidence and operates when phase coincidence occurs for a predetermined minimum time equal to the relay operate time. It is to be noted that, for a fault within the intended reach of the relay, phase coincidence is greater than 1/4 cycle or 4.167 ms so that the operate time is ordinarily set at 4.167 ms. Also, it is to be noted that, to those skilled in the art, the polarizing signal preferably comprises the quadrature voltage shifted 90.degree. leading, V.sub.BC 90.degree. so that the shifted voltage is in phase with the prefault voltage of the faulted phase. The combination of this polarizing signal and the operating signal produces a "variable" MHO characteristic. That is, a characteristic whose diameter is a function of the source impedance behind the relay and the relay reach setting. Such a characteristic is shown in FIGS. 1B, 1C. FIG. 1C shows an advantage of the "variable mho" characteristic is its adaptability to changing arc resistance R.sub.F. This type of relay is termed a quadrature polarized ground distance relay. The advantage of the quadrature polarized ground distance relay is its excellent coverage of fault resistance for a single line to ground fault.
Although quadrature polarized static ground distance relays of the type shown in FIG. 1A are successful for many applications, such relays do present some security problems. One such problem is the difficulty in establishing through calculations the performance of the "unfaulted phase" unit on faults involving ground (particularly in the presence of heavy load transfer) in order to establish relay settings which will prevent misoperation. Another problem is the possibility of overreach on single line to ground, or double line to ground, faults with substantial ground fault resistance and adverse load flow.
Accordingly, it is a general object of my invention to provide an improved static ground distance relay employing phase angle comparator operation.
Another object of my invention is to provide a circuit for such a relay wherein undesired relay operation under heavy load transfer conditions is substantially eliminated.
Another object of my invention is to provide a circuit for such a relay wherein unfaulted phases do not see a fault in the non tripping direction when a single line to ground fault occurs on another phase.
Another object of my invention is to provide a circuit for such a relay wherein overreaching on single line to ground faults with high fault resistance is substantially eliminated.
Another object of my invention is to provide a circuit for such a relay wherein overreaching on double line to ground faults with high fault resistance is substantially eliminated.
Another object of my invention is to provide a quadrature polarized static ground distance relay having the combined functional characteristics of a quadrature polarized ground distance relay and a faulted phase selector.
Another object of my invention is to provide a quadrature polarized static ground distance relay having the combined functional characteristics of a quadrature polarized ground distance relay and a compensated zero sequence directional unit.